The production of hypodermic needles or syringes, in particular for medical purposes but also for other applications is shifting more and more in the direction of the so-called disposable or one-time syringes which are used only once or but a few times and then discarded. Of course, such hypodermic syringes must function properly, yet on the other hand they must not be complicated or expensive because of their predominant one-time use. It has become customary to use so-called disposable syringes for this purpose. They have a syringe tube, or needle, of metal which is connected in sealing fashion and mechanically firmly with a fitting made of plastic material. The cylinder of the hypodermic syringe containing the piston is then firmly connected with the plastic fitting either by being pushed on or by plastic sealing under the influence of heat. The instant invention is concerned with the production of such disposable syringes consisting of a metallic syringe tube and a plastic fitting.
With all such disposable syringes whiich include a metal syringe tube and a plastic fitting the tension-proof connection between the syringe tube and the plastic fitting is an indispensable requirement. If the connection is not firm enough, it has happened that the low pressure in a patient's vein propelled the syringe tube with great impulse from the hypodermic syringe into the vein. Then a risky and complicated operation is required, depending on where the syringe tube comes to rest within the human body, leaving aside the fact that also the syringe tube itself, as it is impelled, can cause considerable damage to body organs. On the other hand, leaks can cause loss of the fluid aspired and also present a risk of embolism by air entering into the system. Apart from tensile strength, the syringe tube is to display also pressure strength against being pressed into the fitting.
There are different ways of meeting these demands. The invention relates among other ways to the known one, described in DE-OS No. 17 04 102, of heating the syringe tube by the passage of current to such a degree, for instance, approximately until it is red-hot or to 500.degree. to 600.degree. C. (assumed value, not verified) so that the plastic material of the fitting will fuse together with the syringe tube. In certain analogy with a welding connection, an additional adhesive component may be dispensed with. However, the invention also relates to a mechanical bond of plastic tube and fitting, which bond may be used for pre-fixing the syringe tube in the fitting prior to an adhesion or fusion process on the one hand, or for a general fixation and sealing of the syringe tube in the fitting on the other hand.
Specifically, the invention starts from the applicant's own internal prior knowledge by which the state of the art according to DE-OS No. 17 04 102 was developed further.
With this earlier method the syringe tube first is pushed into the usual receiving bore in the fitting approximately into the position in which the syringe tube is to be connected with the fitting. At the inside of the fitting there is formed a cup facing the main body of the syringe to be mounted later and into which the inner end of the syringe tube projects by a little distance only. It is disputed among experts whether or not this projection by a short distance or a somewhat greater distance is convenient. By no means a great free path length is desired within the fitting so as to avoid the inclusion of air between the inner end of the syringe tube and the wall of the fitting when filling the syringe since this may for example cause the risk of embolism during the later injection of the contents of the syringe into the human body. Thus the space within the fitting before the inner end of the syringe tube is very limited. Therefore a counterelectrode to be applied for resistance heating of the syringe tube was simply placed flush on the free inner end of the syringe tube.
It is also known (DE-OS No. 17 04 102) to have the inner end of the syringe tube end within the central bore of the fitting and to form a central extension on the counterelectrode engaging in the central bore in the fitting and seated on the interior of the syringe tube by a rounded front end face.
On the other hand, the free end of the syringe tube carrying the injection tip outside of the fitting is engaged by a clamping jaw which may be used first to push the syringe tube into the fitting and then to have the syringe tube abut against the counterelectrode in the final position. In the structure previously known to applicant the clamping jaw, at the same time, functions as the second electrode which, together with the counterelectrode, serves to apply welding voltage supplied by a welding voltage generator to the syringe tube for brief resistance heating of the same. Current is pulsed once or several times through the arrangement, depending on the heating technique chosen.
During the passage of the current more or less strong spark erosion takes place between the counterelectrode and the inner end of the syringe tube. Typical erosion lengths are from 0.1 to 0.3 mm. However, under unfavorable conditions the erosion may be much greater, particularly so if welding voltage from the same welding voltage generator is applied during the same process step to a majority or plurality of syringe tubes and the conditions at the individual syringe tubes are not adjusted exacetly to the same values. In the case of the above-described arrangement, therefore, the clamping jaw is moved by a predetermined stroke in the direction towards the fixing counterelectrode during the fusion process proper or the passage of the current. The stroke mentioned may be in the order of 1 mm or more, for example, 3 mm. An adaptation to the actual erosion at the inner end of the syringe tube which is clamped between the clamping jaw and the counterelectrode is effected by having a sliding grip between the clamping jaw and the syringe tube in friction lock so that the syringe tube can slip more or less with respect to the clamping jaw during the welding process.
Furthermore, at the side facing the fitting, the clamping jaw is narrowed in funnel shape so that individually supplied syringe tubes can be introduced conveniently into the clamping jaw until the respective syringe tube adopts its proper position in the clamping jaw and the clamping jaw can be closed. This requires a distance of a few millimeters between the exit of the syringe tube from the fitting to the place at which the electrical contact is established between the clamping jaw and the syringe tube. This arrangement provides an inaccurate definition of the place of current transfer between the clamping jaw and the syringe tube, a place which changes during the welding process, and, finally, a relatively great distance between the place of entry of the syringe tube into the fitting and the place of the actual current transfer between the clamping jaw and the syringe tube outside of the fitting. This may cause irregularities in the conditions of fusing because of an insufficiently defined resistance path. Furthermore, with some metals of the syringe tube this may cause optically undesirable annealing colors on the free end of the syringe tube outside of the fitting because of an annealing of the syringe tube which did not become effective for the fusion process. Finally, it may also make the metal of the syringe tube undesirably brittle.